The invention relates to the field of wind power engineering and in particular to horizontal axis wind turbine design. Modern wind turbine power output has been increasing with the use of faster turning, larger diameter rotors on higher towers, all pushing the limits of practical manufacturing, transportation and construction. At the same time, the potential for smaller distributed energy sources beckons for new and better solutions. However, in almost a century, little has changed within the theory, formulas or algorithms used to design and optimize wind turbine configurations.
A detailed study within the technical literature available on the subject of wind turbine design reveals that most wind turbine design and optimization is based on 19th century Froude's actuator disc momentum theory. Froude's actuator disc theory uses an incorrect solution to the momentum equation to derive results that diverge from the characteristics of real airflow. The primary result of Froude's momentum theory is that the velocity through the turbine is said to be equal to the average of the free stream and final velocity of the wake. This is an impossibility as the result implies that when the velocity through the turbine is slowed to one half of the free stream velocity then the resulting velocity far downstream would have to equal zero. This implies an infinite expansion of the wake for these conditions. So in fact, this historic momentum theory is both inaccurate and invalid for use in understanding the flow through a wind turbine.
Furthermore, the conventional theories incorrectly make use of Bernoulli's equation to compare the pressure differential across the wind turbine to the loss in kinetic energy of the final airflow. Bernoulli's equation is an over-simplification of the energy equation that assumes a steady flow of an incompressible fluid at a constant temperature. The change in energy that occurs at the rotor of the wind turbine is a shift from internal thermal and pressure energies to rotational kinetic energies and cannot be handled by Bernoulli's equation. The only way to analyze this exchange accurately is by using the total energy equation including the enthalpy terms relating pressure and temperature. This combination of incorrect solution to the momentum equation and misuse of Bernoulli's equation results in the derivation of the Lanchester, Joukowsky, and Betz limit, which incorrectly claims that only 59.3% of the relative value of kinetic energy can be extracted from the airflow.
Wind turbine design also is based on the early 20th century work of Hermann Glauert and others working on aircraft propeller and rotorcraft design theory. Their work resulted in what is known as blade-element momentum theory, which is fundamentally used still today. However, the purpose of a propeller is fundamentally different from a wind turbine. A propeller must convert shaft power into thrust. If the propeller also causes the slipstream to rotate, then this is correctly considered an inefficiency. The purpose of a wind turbine is to convert thrust into shaft power. It has been a common misconception that if the wind turbine causes the slipstream to rotate that this too is a similar inefficiency. Nevertheless, this rotation is also a necessity unless the turbine uses stator vanes, which is not the case. A rotating wind turbine cannot extract power from the wind unless it imparts an equal and opposite torque into the slipstream and therefore causes the slipstream to rotate. The Glauert method of accounting for rotation incorrectly assumes that the slipstream rotation is directly proportional to turbine rotation, as it is with a propeller. However for a wind turbine, the slipstream rotation is in the opposite direction and inversely proportional to the turbine rotation. None of the current methods available have properly accounted for the effect of these rotational parameters within the corrected energy equation.
Furthermore, conventional methods often and incorrectly relate the thrust force times velocity as equal to the power extracted; however, those methods fail to account for the condition of a free spinning rotor or propeller. When a rotor or propeller is free spinning with no torque applied to its connecting shaft, it is still creating a very significant negative thrust but with no power exchange other than frictional losses. This fact should dismiss the assumption that thrust times velocity equates to power out, which is a basis of many conventional theories.
Other design trends in horizontal axis wind turbines include ducted, shrouded, and diffuser augmented turbines, all of which can be referred to as constrained flow rotors. Although these constrained flow rotors show promise, their commercial success has yet to be realized; their basic designs still rely on the same theories as the open rotors and these current theories improperly reward diffuser designs and incorrectly discourage highly loaded multi-bladed wind turbines.
In fact, the historical theoretical basis of wind turbine design has become ingrained with assumptions, mistakes, and misconceptions. The consequence of these historical errors is that the importance of the rotational factors have been neglected and the modern wind turbine design has been improperly skewed towards the conventional three-bladed wind turbines that we see in the field today. The conception for the thermodynamic wind turbine presented here came as a result of the desire to correct the aforementioned flaws within the conventional wind turbine design theories.